The present invention relates generally to permanent magnets, and more particularly relates to permanent magnet structures.
Recent developments utilizing magnetic fields often require fields having very high strengths, i.e. high flux densities. The flux densities needed for these applications frequently exceed 1 to 2 Tesla (T). For example, solid magnetic refrigerants exhibit the magnetocaloric effect in the presence of magnetic fields of 1 T and above, and MRI machines employ magnetic fields having flux densities around 2 T. In order to obtain high flux densities above 1 or 2 T, electromagnets have almost exclusively been used. It is also common to employ a superconducting solenoid which greatly reduces electrical resistance and provides a high powered, superconducting magnet.
Unfortunately, electromagnets require large power supplies for charging and superconducting magnets require extensive cooling systems to maintain the solenoid below certain critical low temperatures. Liquid helium is typically used and must be replenished periodically to cool the magnet, which makes the magnet inherently large and expensive. Not only do these attributes increase the cost of high powered electromagnets, but they also diminish, if not eliminate, the portability of electromagnets due to their large size and weight, especially those capable of generating very strong magnetic fields.
Permanent magnets offer an alternative magnetic flux source to electromagnets, and do not require large power supplies or cooling systems. Nonetheless, permanent magnets in the past have been unable to generate magnetic flux densities commensurate with electromagnets. Recent advances in permanent magnets, however, have greatly increased the magnetic flux densities of permanent magnets. For example, the use of rare-earth metals such as Neodymium (Nd) and Samarium (Sm) have increased the strength of permanent magnets. The most widely used permanent magnets currently are Ndxe2x80x94Fexe2x80x94B and SmCo5. Furthermore, arrangement techniques employing these materials have produced permanent magnets that can produce magnetic fields having flux densities of 2 T.
With regard to permanent magnet arrangement techniques, the underlying concept is to construct arrays of magnet segments in a fashion whereby their magnetization vectors are aligned and cooperate to create a coherent magnetic field of greater strength. In other words, the flux density of the combined field is greater than the residual flux density (remanence) of the magnetic material itself. For example, a xe2x80x9cmagic ringxe2x80x9d, also known as a hollow cylinder flux source (HCFS) is a conventional configuration that combines as few as 8 magnetic segments in a disc or tube-like structure that defines an opening or gap through the center. The magnetization vectors are aligned to create a magnetic field in the central gap having a higher flux density than the residual flux density of the magnet segments. Similarly, a xe2x80x9cmagic spherexe2x80x9d, also known as a hollow spherical flux source (HSFS) comprises an array of magnets constructed in a spherical configuration, leaving a gap at the central of the sphere. A small bore is provided through the sphere for access to the center gap, where a high flux density is created.
HCFSs and HSFSs employing rare-earths have been constructed to generate strong magnetic fields having high flux densities per net weight of magnetic material. While these permanent magnets provide benefits over superconducting electromagnets in terms of weight, cost and mobility, their mass production would require an enormous consumption of rare-earths. Further, rare-earths are very expensive compared to iron, nickel and cobalt. It would therefore be ideal to use a magnet array structure that maximizes the flux density generated by a given weight of magnetic material. The cost reduction allowed by constructing the magnet structures with the least amount of material for a given field strength will be extremely important.
In light of the above, a general object of the present invention is to provide a permanent magnet flux source that maximizes the flux density generated per weight of the magnetic material.
In that regard, it is also an object of the present invention to provide a new permanent magnet array structure whereby higher flux densities are created without the need for additional or new magnetic material.
In accordance with these objectives, a permanent magnet structure has been provided for maximizing the flux density per weight of permanent magnet material comprising a hollow body flux source for generating a magnetic field in the central gap of the hollow body. The magnetic fields within the hollow body flux source are oriented to generate a coherent and uniform magnetic field in the central gap having a flux density greater than the residual flux density of the fields within the hollow body flux source. The hollow body flux source has a generally elliptic-shape, defined by unequal major and minor axes. The major axis is disposed generally parallel to the magnetic flux lines generated in the central gap. This particular structure results in a magnetic field in the center gap having a flux density greater than comparable xe2x80x9cmagic cylindersxe2x80x9d or xe2x80x9cmagic spheres,xe2x80x9d where the major and minor axes are equal.
It is a feature of the present invention to provide an elliptic-shaped hollow body flux source in the form of an elongate tube wherein the cross-sectional shape of the tube is elliptical. Similarly, the elliptic-shaped hollow body flux source may take the form of an ellipsoid or ovoid. These elliptic-shaped structures exhibit the unequal major and minor axis which result in a higher flux density at the center gap of the hollow body while minimizing the amount of magnetic material used.
It is a further feature of the present invention to provide the hollow body flux source with an insert of soft magnetic material in the central gap, and a shell of soft magnetic material surrounding the hollow body. The soft magnetic material has high permeability such that the insert and shell reduce the magnetic flux leakage and focus the flux density lines in the central gap.